This invention relates to flowmetering equipment and, in particular, to a flowmeter utilizing a vibratory sensor arrangement particularly suited to measure the flowrate of heterogeneous fluids, e.g., fluids such as steam having constituents in both vapor and liquid phases.
Various and sundry arrangements have been proposed by those skilled in the art which exploit the phenomenon of the Karman vortex street in order to measure fluid flowrates. More than a decade ago W. G. Bird (U.S. Pat. No. 3,116,639) devised an arrangement in which a pivoted vane-like element was positioned downstream of a vortex shedding body. Flowrate was measured by detecting the frequency of pivotal oscillation of the element. In general, the Bird arrangement, and others like it, suffered from a basic problem: intermittent vortex shedding. This problem perplexed the art until A. E. Rodely (U.S. Pat. No. 3,572,117) recognized that a suitable dimensioned bluff body, having a base surface facing fluid flow and downstream surfaces to control oscillatory flow, would produce vortex shedding free of intermittency. In this arrangement, a temperature sensitive sensor situated outside the wake generated by the bluff body detects vortex shedding and produces electrical pulses proportional to the flow rate. The sensor is mounted separately in the pipeline wall and the probe-like sensor is positioned in the low turbulence flow zone outside of the wake.
Improved versions of the Rodely bluff body flowmeter generally detect vortex shedding in the high turbulence flow zone immediately downstream of the base surface of the bluff body. Thus, in U.S. Pat. No. 3,732,731 a removable temperature sensor, on end of a rod-like holder, is located at the intersection of two channels in the bluff body. One channel extends between the downstream surfaces and the other, into which the holder and sensor are inserted, extends along the long axis of the body to the exterior of the conduit. In another arrangement, shown in U.S. Pat. No. 3,796,095 the two channels are in non-fluid-flow communication with one another, a cylindrical body containing a ferromagnetic disc in situated in the one channel which extends between the downstream surfaces, and a magnetic detector is situated in the other channel. As the disc moves, it interrupts a magnetic field causing perturbations which result in an EMF related to the flowrate. Finally, in application Ser. No. 321,532 filed on Jan. 5, 1973, and assigned to the assignee hereof, the bluff body has a pair of orifices proximate the downstream surfaces, a cylindrically shaped chamber within the body, and a shuttle ball free to move a relatively short distance within the chamber along the long axis of the bluff body in response to vortex induced pressure changes at the orifices.
While the foregoing flowmeter arrangements represent significant contributions to the state of the art, none has generally been suitable for measuring the flowrate of heterogeneous fluids: those containing constituents in both vapor and liquid phases, especially steam. In particular the high temperature of steam, often in the neighborhood of 500 degrees F., had deleterious effects on flowmeters utilizing thermistor sensors, whereas the corrosive, errosive, non-lubricating characteristics of steam tend to clog flowmeters utilizing shuttle cylinders and shuttle balls.
It is therefore one object of my invention to provide a flowmeter capable of measuring the flowrate of heterogeneous fluids.
It is another object of my invention to measure the flowrate of corrosive, errosive, non-lubricating fluids.
It is yet another object of my invention to measure the flowrate of steam.
Other schemes, which however do not utilize Rodely bluff bodies, have been suggested for controlling the condition of vortex formation. In particular, M. Tomota et al (U.S. Pat. No. 3,564,915) teach a rod-shaped object for producing vortices, the object having a transverse bore the ports of which open in the vicinity of the separation points of the boundary layers of the fluid from the object. Various types of sensing elements can be positioned in the bore. For example, in FIG. 7A, a stainless steel diaphragm 22 is used as a sensor, and at column 9, lines 66-74, it is stated that flow rate can be measured by detecting "the vibration of the diaphragm in the form of resistance variation with a strain gauge attached to the diaphragm or by converting the vibration of the diaphragm into an electric signal in the form of an electrostatic capacity change or electromagnetic change or by directly detecting the vibration of the diaphragm."
Generalized configurations such as those shown in the Tomota et al patent fail, however, to address significant design problems which render flowmeters practically useable in terms of measurement accuracy as well as field serviceability. From the standpoint of accuracy, these prior art proposals recognize neither resonant frequency problems nor fluid drainage problems associated with the sensor chamber. On the other hand, when considering fluid serviceability, the latter prior art proposals are not designed so that the most vulnerable part, the sensor, is easily replaced in the field in order to reduce flowmeter downtime.
It is therefore still another object of my invention to provide a Rodely-type flowmeter utilizing a vibratory sensor arrangement in which accuracy of measuring the flowrate of heterogeneous fluids such as steam is enhanced by facilitating liquid drain-off from the sensor.
It is another object of my invention to provide a Rodely-type flowmeter utilizing a vibratory sensor arrangement in which the sensor chamber has no resonant frequency near to the vortex shedding frequency.
It is also an object of my invention to provide a Rodely-type flowmeter utilizing a vibratory sensor arrangement in which field serviceability is enhanced by situating the sensor so that it is easily replaced in the field.